Nickel-based and cobalt-based superalloy materials are commonly used to provide high mechanical strength for very high temperature applications, such as for the blades or other components of a gas turbine engine. Such components are very expensive, and thus the repair of a damaged part is preferred over its replacement. However, known weld repair techniques for superalloy materials have met with only limited success, due primarily to the propensity of superalloy materials to develop cracks during such welding operations. In addition to hot cracking of the weld filler metal and heat affected zone, these materials exhibit strain age cracking, which results in cracks in the base metal of the component.
Several techniques have been proposed to improve the weldability of superalloy materials. U.S. Pat. No. 4,336,312 describes a combination of a controlled chemical modification of a cast nickel-based superalloy material along with a pre-weld thermal conditioning cycle. U.S. Pat. No. 6,364,971 describes a laser welding technique used following a pre-conditioning hot isostatic process. U.S. Pat. No. 6,333,484 describes a welding technique wherein the entire weld area is preheated to a maximum ductility temperature range, and this elevated temperature is maintained during the welding and solidification of the weld. Each of these patents is incorporated by reference herein.
The assignee of the present invention produces gas turbine engines utilizing a variety of materials, including blades formed of a directionally solidified (DS) cast nickel-based superalloy material sold by Cannon-Muskegon Corporation under the designation CM-247 LC. CM-247 LC is known to have the following nominal composition, expressed as weight percentages: carbon 0.07%; chrome 8%; cobalt 9%; molybdenum 0.5%; tungsten 10%; tantalum 3.2%; titanium 0.7%; aluminum 5.6%; boron 0.015%; zirconium 0.01%; hafnium 1.4%; and the balance nickel. Such blades are typically repaired by welding at elevated temperatures, so called hot-box welding, utilizing specially selected filler metal. Hot-box weld repairs may take eight hours or more to complete, and the requirement for working inside of the hot box to maintain the elevated temperature makes it difficult to perform such welds robotically.